A conventional LC/MS is shown in FIG. 2. Components of liquid sample are separated in the column 21 of the LC section 20 and are successively introduced into the interface section 30, where the liquid components are nebulized by spraying and ionized. The ions pass through the desolvation heated pipe 32 placed between the interface section 30 and the mass spectrometric section 40, and are converged and accelerated by the ion lens 41 toward the quadrupole filter 42. In the quadrupole filter 42, ions having a preset mass number (the ratio of mass to charge m/z) can pass through the quadrupole filter 42 and are detected by the detector 43.
In the interface section 30, the liquid component is nebulized and ionized by heating, by high-speed air flow, by high-voltage electric field, etc. An electrospray ionization (ESI) method and an atmospheric chemical ionization (APCI) method are two most prevalent methods of ionization. In the ESI method, a high voltage is applied to the nozzle 31, where the sample solution is separated by electrical charges owing to the high voltage. The sample solution is drawn into droplets (nebulized) by means of the Coulomb attraction and the droplets divide up successively by means of the Coulomb repulsion until they are ionized. In the APCI method, the sample solution is nebulized by heating at the nozzle 31, and the droplets of the sample solution chemically react with ions of a carrier gas (buffer ions) produced by a corona discharge, whereby ions of the sample solution are produced.
FIG. 3 shows the spraying section (the nozzle 31 of FIG. 2) of a conventional electrospray ionizer. A glass capillary 11, which is connected to the outlet of the column 21 of the LC section 20, is inserted into a narrow metal tube 12, and the fore end of the glass capillary 11 extends out of the metal tube 12. The metal tube 12 is held in a nebulizing tube 13 with a certain gap, where a nebulizing gas, such as nitrogen gas, is supplied from the back end (the end toward the column) into the cap. The nebulizing gas blows out from around the fore end of the metal tube 12.
When a high voltage of several kilovolts is applied by the high voltage generator 14 to the metal tube 12, the sample solution in the glass capillary 11 is electrically charged and is sprayed out from the end of the glass capillary 11 into tiny droplets with the aid of the nebulizing gas. The solvent in the electrically charged droplets evaporates while the droplets contact with the ambient gas, whereby the ions of the sample are produced. Though the spraying and ionization of the sample solution can occur owing to the Coulomb force alone without using the nebulizing gas, the nebulizing gas helps to promote a stable production of a large amount of ions.
When the number of ions produced in an electrospray ionizer is to be increased, several conditions should be appropriately adjusted to produce finer droplets, among which the voltage applied to the metal tube 12 is included. In the electrospray ionizer of the above structure, the strength of the electric field at the discharge end (fore end) of the glass capillary 11 depends largely on the length of the extension d of the glass capillary 11 from the metal tube 12. It is therefore important to adjust the extension d to such length at which the number of ions produced reaches a maximum.
When, however, ions are being produced, or when the sample solution is being nebulized, the high voltage is applied to the metal tube 12, so that the operator cannot touch it. Conventionally, therefore, the extension length d is determined appropriately beforehand, and then ionization is performed. This inevitably leads to a poor adjustment or a longer adjusting time.